Discard half of the starter, keeping about 60g in the container. Add the all-purpose flour and water to the container, and stir until well combined. Mark the level of the mixture on the side of the container with a rubber band or piece of tape. Loosely cover and let it rest in a warm spot for another 24 hours.
Day 3: Check the starter. It should be very bubbly and risen, almost doubling in volume. If you do not see much activity or the starter hasn’t increased in volume, continue to let the mixture sit until it does. (This could take another 24 hours.)
Once risen, discard half of the starter and feed again with the same amounts of flour and water. Loosely cover and let the starter sit until it has doubled in size. This could be anywhere from 6 to 24 hours.
Day 4/5: Continue discarding and feeding the starter 2 more times after it doubles in volume each time. (You will be creating a bit of discard or waste in this initial phase, but it’s worth it! Once your starter is healthy and active, you can use the discard in other recipes.)
After the final feeding and rise, your starter is ready to use! You can follow my recipe for homemade Sourdough Bread to put it to use.
To store your starter long-term, keep it refrigerated so it stays healthy. Before storing, discard and feed one more time. Let the starter sit in a warm spot just until it starts to rise (about 2 hours), then cover tightly and refrigerate. Feed it by repeating the same process of discarding and adding flour and water once every 2 weeks to keep the starter healthy. The cold will slow the yeast down so that they feed on the flour more slowly. When ready to bake again, discard and feed as normal. Let the starter sit in a warm place until doubled in size. Use as directed in the recipe.